Fluid power steering gear



March 12, 1957 F. w. DAVIS FLUID POWER STEERING GEAR Filed Aug. 25l 1955 iwf ff'r

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United States Patent O rFLUID POWER STEERING -GEAR Francis W. Davis, Belmont, Mass.

Application August 25, :1953, Serial No. 316,327

2 Claims. '(Cl. I4-388) This application `is a continuation-impart `of my copending application Serial No. 225,849, led May ll, 1951. The invention relates to ya iluid power steering Ygear including a servo-motor of the vtype described and `illustrated in U. S. Patent No. 2,410,049 granted .to me October 29., 1946. The uid motor comprises a cylinder `which is a part of the steering column and 4is coaxial with the steering post, and a piston slidably mounted in the cylinder, fluid under pressure being admitted into either end ofthe cylinder to press against the corresponding end ofthe piston to cause the .piston to slide in Ithe desired direction. One of the objects of the present invention is to provide a 'three-part piston, Ithe mid part of which is a nut which has .a rack on its -outer surface to mesh with a gear sector. This mid part .has a oating connection with the end parts so that no transverse force is impressed on the latter. The nut is in threaded `engagement with a steering post which extends therethrough, andthe gear sector with `which lthe rack meshes is on a `cro-ss shaft which may be connected in the usual manner to the `vehicle wheels .to be detlected to direct the course of the vehicle. Power iluid, preferably a liquid such as oil, is supplied in the usual manner lto `a control valve which is in the steering column. The ysteering post, which is manually rotated to steer the vehicle, is at the axis of the column and extends down through the control valve as `well as the piston. Rotation of :the steering post thus results in axial movement `ot the piston which in turn `results `in rocking movement of the Ycross shaft. The reactive axial thrust :on the steering post trom the piston when the steering post is rotated, tends to move the `post axially. The post is capable of 4such movement for a small distance in `either direction lfrom a neutral position, such axial movement-of the post .being opposed by suitable centering means which yieldingly tend to prevent axial movement of the post in either direction from its normal position relative to the casing of the steering column. The control valve for the motor consists chiefly `of itwo cooperating members one of which is stationary and may Vbe a part of the steering column casing itself, the other or inner member bein-g nested and axially `slida'ble in the outer member a limited -distance to vary the several valve ports formed by grooves in the two members. The inner valve member is moved by axial movement of the steering post.

The centering means hereinbefore referred to opposes axial movement of the steering post and of the movable valve member with respect to the stationary valve member unless and until the driven part (such as the crossshaft) otters suflicient resistance to movement in response to torque exerted on the steering post to overcome the opposition of the centering means, whereupon the steering post moves axially and the valve is thus operated to direct the power iluid into one end or the other of the cylinder so as to produce axial follow-up movement in the piston which will restore the steering post and movable valve member axially to their neutral positions. Thus the steering mechanism is a manually operated 2,784,61 Patented Mar. l2, 1957 steering Lgear until resistance `to the steering effort exceeds a predetermined value, whereupon power assistance is instantly and automatically called into action. When the valve-centering `means :is actuated by springs only, the `magnitude of the resisting `force which must be exceeded to bring fthe power actuation into `play depends on the physical `characteristics of the springs. The centering effect of the springs may vbe-augmented kby Vpressure derived from 'the `power fuid entering the control valve. A centering means of this *kind isdescrihed in lmy Patent No. 1,937,470, granted November A28, 1933.

According to Vthe present invention, the filuid pressure which is built up inthe supply line from the pump when power is called into -play in response to excessive road resistance Ais also lutilized to oppose and balance the lateral thrust on the mid part of the piston resulting from a camming eect `of the `gear sector which meshes with `the rack teeth on said `mid part.

For a `more vcomplete understanding of the invention, reference may be had to the following description 4of certain embodiments thereof, and to the drawing, of which- Figure 1 is a vfragmentary sectional View of a steering mechanism embodying the invention;

Figure 2 is a section on the line 2-2 of Figure 1;

'Figure 3 is a sectional view of a modied form of the invention; and

Figure 4 is -a section `on the line 4-4 of Figure 3.

Figure l shows in section the lower portion of a steering column rof an automobile, embodying the invention. At the lower end of the `column is a casing member 10 which serves as the cylinder of a iluid motor. A piston -12 is slidably 'tted in the casing member 10, this piston forming chambers 14 and 16 within the cylinder between the ends of the piston and the respective ends of the casing. The piston 1.2 is `composed of three members 22, 24, and 26. The members 22 and 26 are end members and are secured to the central member 24 by suitable studs 28 and nuts 30 as indicated in Figure 2. The studs pass through yholes in the central member 24 which are large enough to leave a little clearance, and are screw threaded into holes in the end members '22 or 26. The nuts 30 are set up tight enough to `insure that the three parts reciprocate as a unitary piston but not `so tight as to prevent transverse self-adjustment of the center member 24 which is thus allowed to oat with respect to the end members. The end members 22 and 26 are provided with packing rings 31 to prevent leakage from the chambers 14"and 1f6 along the outer surface of these members.

The center member 24 is a nut having a central bore, and is operatively related to a steering post 32 which extends downlthrough the piston coaxially with the casing 10, `the connection being of the worm and nut type. In order to minimize friction, `the interfaces of the nut 24 and the steering post 32 are rhelically grooved to accommodate `a series -of ball bearings 3d which .um in these grooves when the steering post is rotated.. A return tube 36 is secured to the mid member by a strap 38 so as to transfer the balls which approach one end of the grooved portion of the steering post to a point near the other end of the grooved portion of the steering post. in the embodiment of the invention shown in the drawings, the nut 24 is a ball nut. T his nut is externally rectangular, as indicated in Figure 2, a series of gear `teeth 4t) being f formed on one face thereof, these teeth constituting a rack which meshes with a gear sector 42. This gear sector is secured to or is integral with a cross-shaft which is connected to the vehicle wheels in such a way as to deilect them when it is rocked in one direction or the other.

The portions of the steering post 32 immediately below and above the nut 24 are tted in tubular bearing members 50 and 52 which project toward each other` and are integral respectively with the cylinder ends 18 and 2t). The end members of the piston are slidably tted on the outer surfaces of the bearing members Sti and 52, and packing rings 53 are provided to prevent leakage along these surfaces from the chambers 14 and 16.

When the steering post is `turned for the purpose of deecting the vehicle Wheels, the worm connection between the post and the ball nut 24 results in a reaction force having an axial component which tends to move the lower portion 32 of the steering post up or down in the direction of its axis according to the direction of rotation of the post. Suitable yielding means are provided to resist such axial movement of the post. If the resistance to steering effort overcomes the force provided for opposing axial movement `of the post, then a control valve is operated in such a manner as to direct power l'luid into the chamber 14 or lthe chamber 16 so as to move the piston axially in a direction to restore the post 32 to its normal position.

The control valve by which the power lluid is directed into one or the other of the chambers 14 and 1.6 is partly shown in Figure l and is more completely shown and described in the said Patent No. 2,410,049. The movable valve member is a hollow cylinder 66 having three circumferential grooves on its outer surface which lap and communicate with cooperating grooves on the inner surface of the casing 60 in which the valve member is slidably tted. The middle groove 72 in the valve member is constantly in communication with lthe pump (not shown) which supplies the power tiuid. Hence when the power mechanism is called upon to operate, ythe pressure of the uid in the groove 72 builds up and is communicated to the chamber 14 or 16 by passages not shown.

When power is applied to overcome a road resistance of considerable magnitude, there is a correspondingly grealt interfacial pressure between flanks of the mutually engaged teeth of Athe gear sector 42 and the rack 40, which pressure has a substantial component transverse to the axis of the steering post 32. The purpose of the tubular bearing members 50 and 52, according to the invention, is lto take this thrust and prevent the piston l2 from bearing heavily against the inner wall of the cylinder 1t) as it moves back and forth. Furthermore, according to the invention, an arrangement is provided whereby the transverse thrust on the piston is opposed and approximately balanced so as to minimize wear on all the bearing surfaces within the cylinder.

For this purpose the luid pressure which builds up in the inlet groove '72 when road resistance is encountered is utilized. Such pressure is instantly communicated through a passage 80 to a chamber 82 where it acts on the head of a dalt auxiliary piston 84 which is preferably provided with the usual sealing ring 86. The piston 84 bears `on a spacer S7 which in turn bears on the side of the nut 24 opposite to the gear sector 42, thus counteracting the component of thrust on the rack teeth 40 which is transverse to the axis of the steering post 32. Since the forces on the teeth 40 and on the Vpiston 84 both result from lthe same liquid pressure, they can be made to balance each other approximately by designing the piston 84 with the appropriate area of surface exposed to liquid pressure.

A modified form of this feature of the invention is shown in Figures 3 and 4. In this form the nut 24 has sloping cam surfaces engaged by four cam elements 92 carried by a plate 94, two at each end of the plate. Aligned with the cam elements are four plungers 96 which slide in bores in the piston parts 22 and 24 which are in communication with one or the other of the chambers 14 and 16. When liquid pressure builds up in either of these chambers, it presses on the corresponding pair of plungers 96 to tend to displace the platte 94 longitudinally and thus to exert transverse force on the nut through the cams 92 to oppose the transverse components of the thrust on [the rack teeth 40.

It is desirable to avoid as far as possible any backlash or looseness in the steering connections, including the meshing of the rack teeth with the gear sector. For this purpose the teeth of the rack and sector are slightly inclined with reference to the axis of the cross-shaft, as indicated in Figure 2. Hence, by adjusting the crossshaft axially, any slack or back-lash between the gear sector and the ball nut can be eliminated. To hold the cross-shaft in any axially adjusted position, an end of the shaft is recessed and the head of a bolt 98 is snugly retained in lthe recess, relative axial movement between the bolt and shaft being prevented, but relative rocking movement being permitted. The bolt is threaded through a hole in the housing so that rotation thereof results in axial movement of the shalit 44. A lock nut 100 retains the bolt in position of adjustment.

I claim:

l. In a lluid power steering gear, a cylinder with a chamber at each end for power duid, a piston reciprocable in said cylinder, said piston having a mid portion comprising a nut with rack teeth on one side thereof, a gear sector with teeth meshing with said rack teeth, said cylinder having a lateral opening through which said gear sector extends, a cross shaft on which said gear sector is mounted to be rocked by reciprocation of said piston, and fluid-operated means for exerting pressure ou the side of said nut opposite to the rack teeth, whereby to oppose the transverse component of force on the nut resulting from the camming effect on the mutually engaged flanks of the rack teeth and gear sector teeth when fluid power is applied through the piston to rock the cross shaft.

2. Apparatus as in claim l, said duid-operated means including an auxiliary piston opposite said nut and reciprocable transversely to the axis thereof, and means directing against said auxiliary piston uid from the source supplying uid under pressure to one of said chambers.

References Cited in the tile of this patent UNITED STATES PATENTS 

